Add DbContext Validate to CLI/PS commands

[julielerman]

The validation that EF does when you run a migrations command provides very helpful error messages.

Is it possible to have a command just for that task so you don't have to add a migration just to check to see if the model is valid?

CLI
dotnet ef dbcontext validate

Powershell
validate-dbcontext (follows pattern of scaffold-dbcontext)

[bricelam]

Hmm, we have dotnet ef dbcontext info, but no PMC equivalent.

[bricelam]

Then again, it never builds the model. ...but we could change that.

[bricelam]

#9096 Adds Get-DbContext in addition to dotnet ef dbcontext info. These commands will run model validation and print out information about the context (provider name, database server, DbContextOptions, etc.)

There is a small bug in the way we log (#8350), but if you're using an ASP.NET Core project, everything will work as expected.